In some vehicles, unstable driving performance can occur in some operating situations during the transition from the overrun phase or from the idle phase into the traction phase (drive phase) of a drive unit of a motor vehicle. In the extreme case, this can lead to an oversteering of the vehicle.
From U.S. Pat. No. 5,048,482, it is known to provide a time-dependent increasing limit rpm at the transition to improve the same. This limit rpm cannot be exceeded by the rpm of the drive unit. In this way, the transition in the traction phase is improved in several situations especially for a fault in the area of the air throughput to the adjusting device influencing an engine.
After ending an overrun operation, when pressing the accelerator pedal, it is known from U.S. Pat. No. 4,311,123 to increase the fuel quantity supplied to an engine in accordance with a selectable time function. Here too, a smooth transition into normal driving operation is of primary importance. However, measures which avoid a possible occurrence of unstable driving performance are not described for the transition from the overrun phase of operation or idle phase of operation into the traction phase.
It is an object of the invention to provide measures with which an unstable driving performance is avoided which occurs during the transition into the traction phase of a drive unit.
The method of the invention is for controlling a drive unit of a motor vehicle and includes the steps of: detecting a variable representing the torque of the drive unit; controlling the variable on the basis of a command of the driver of the motor vehicle; detecting the slip of at least one drive wheel in one of the following operating phases: overrun phase, idle phase and braking phase; determining a limiting value for the variable in dependence upon the slip; and, limiting the variable to the limiting value at or near a transition from one of the operating phases into a traction phase of the drive unit.
The procedure of the invention of limiting an operating variable during the transition from overrun operation or idle operation into traction operation (load change) in dependence upon the drag slip and/or brake slip during the overrun phase reduces the danger of instability during a load change because the driver cannot now use the entire engine torque. The operating variable which is limited determines the torque of the drive unit. In this way, the danger of oversteering the vehicle because of the instability is reduced. An overall improvement of the driving stability of the vehicle during a load change is the result.
The limiting of the torque is especially advantageous in systems wherein disengagement of the clutch takes place in the overrun operation and wherein engagement of the clutch takes place again when the accelerator pedal is depressed. In control concepts of this kind, the then occurring instability during reengagement is considerably reduced.
Corresponding advantages are also attained when, in the overrun mode of operation, an anti-blocking controller is active or a braking phase is present. In this case, the braking slip during the overrun phase or braking operation is the starting point of the limiting value.
It is especially advantageous that the limiting value is increased in accordance with a time function so that the total torque of the drive unit is again made available to the driver little by little and, in this way, the dynamic of the vehicle is unaffected.